In recent years, it has become apparent that conventional diesel combustion cannot alone meet emission levels mandated for the future. Hence, diesel engine manufacturers have been considering multiple combustion modes as a means to reduce emissions. Alternate combustion modes such as homogeneous charge compression ignition (HCCI), low temperature combustion (LTC), and premixed charge compression ignition (PCCI) are being developed and implemented on diesel engines, together with conventional diesel combustion.
At steady-state, alternate combustion modes offer great potential to reduce engine emission levels. However, because the applicable speed-load regions of different combustion modes are different from each other, the engine must seamlessly switch among these modes.
The different combustion modes are achieved by different fueling and in-cylinder conditions. Some modes are close to the edge of unstable combustion, and are very sensitive to engine conditions.
For diesel engines, fueling control can be exercised precisely on a cycle-by-cycle basis. However, in-cylinder conditions change at a much slower rate (over several combustion cycles). Poor control over in-cylinder conditions not only diminishes the merits of alternate combustion modes but also worsens drivability and emissions.